CA1062802A - Signal processor for ultrasonic imaging - Google Patents

Abstract

PATENT APPLICATION
of WILLIAM L. BEAVER
for SIGNAL PROCESSOR FOR ULTRASONIC IMAGING
ABSTRACT
A signal processor for an ultrasonic imaging system permits the selection of scan angles and focusing distances.
The system includes an ultrasonic receiver comprising an array of electromechanical transducers, with the individual transducers being coupled to phase selection circuitry whereby non-continuous delay values can be introduced between adjacent transducers. Proper selection of the delay values between adjacent transducers can accomplish preferential ultrasonic reception or transmission in particular directions. An optimized switching arrangement minimizes the number of electronic components required to provide the desired delay values.

Description

106Z~30Z
BACKGROUND OF THE INVENTION
Field of the Invention This invention is a further development in the field of direction selecting for ultrasonic imaging systems. In particular, the invention is concerned with the introduction of discontinuous delay values between the various elements of an array of transducers, and with an optimized switching arrangement for minimizing the nurnber of electronic components required to provide the required delay values for steering and focusing of the ultrasonic imaging system.
Referring to the drawings:-FIG. 1 is a prior art sketch illustrating theimpingement of an ultrasonic wave front upon an array of transducers in an ultrasonic imaging apparatus, FIG. 2 illustrates in block-diagram form one particular ultrasonic imaging apparatus known to the prior art, FIG. 3 illustrates in block-diagram form another particular ultrasonic imaging apparatus known to the prior art, FIG 4 illustrates in block-diagram form an ultrasonic apparatus according to one embodiment of the present invention and F~G. 5 illustrates in block-diagram for an ultrasonic imaging system according to one embodiment of the present invention.
Description of the Prior Art In a pulsed ultrasonic beam imaging apparatus, a particular scanning angle and focal distance for an array of electromechanical transducers can be obtained by pulsing each of the transducer elements of the array in a proper timing sequence, so that the acoustic pulses transmitted from each of the transducer elements all arrive at the desired focal point at the sc~me instant in time. This principle is illust-ated schematically in FIG. 1 ~here the transducer array 10 consists of an unspecified num~er of individual transducer elements, Wlt~ t~e element on the left end being indicated by the reference number 11, the element on the right end being indicated by the reference num~r 20, and intermediate elements of the array being indicated by the reference num~ers 12,..., 15,..., 19. The element -15 represents the transducer element disposed at or near the center of the array. In order to focus an acoustic pulse at the point P which is located at a distance from thé trans-ducer array 10, it is necessary that the pulses transmitted from the individual transducers all arrive at ~oint P at the same time. Thus, ~he acoustic pulse from the left-end transducer 11 must travel to a point B on the pulse path to the focal point P before the right-end transducer element 20 is excited to emit an acoustic pulse. By delaying the electrical excitation of each of the transducer ele~ents to the right of the end element 11 by an appropriate amount, it is possible to bring the pulses from all the transducer elements simultaneously to a focus at point P. The focal point P is identified by the focal distance f from the center element lS to the point P, and by the angle ~ between the normal to the array at the center element 15 and the path from center element 15 to point P.
Similarly, in order to operate the transducer array 10 in a receiving mode so as to focus upon the point P as a source of reflected acoustic energy, it is necessary that, as a wave front reflected from the point P impinges in turn upon each of the transducer elements of the array, the electronic signals thereby generated by each of the transducer elements in ~uccesSion be detected sLmultaneously by a receiver. For example, as seen in FIG. 1, a signal reflected from the point P ~ill arrive simultaneously at the right-end transducer element 20 and at the point B on the path from point P to the left-end transducer element 11.
Therefore, the electronic signal produced by the.right.-end transducer element 20 when operating in the receiving mode must be delayed during the time interval required for the acoustic wave front travelling along the path from point P to the left-end transducer element 11 to travel the distance from point B to element 11. The electronic signals generated by the intermediately disposed. transducer elements - of the array 10 must like~ise be delayed by s~itable intermediate time intervals before being combined so as to provide a coherent image of the point P.
Various techniques have been used in prior art imaging systems for obtaining coherent delays between the individual receiving elements of a transducer array in order to provide an electronic analog image of the source of reflected waves. One such prior art technique is shown in FIG. 2 where the transducer elements 11, 12,... , 20, representing an unspecified number of transducer elements, are arranged in a linear array with the left-end element being indicated by reference number 11 and the right-end element being indicated by the reference number 20. Each transducer element is coupled through a separate variable delay line 21, 22, ..., 30, respectively, to a transmit/
receive unit 31. The transmit~receive unit 31 is programmed to transmit electrical pulses to the individual transducer elements for conversion into acoustic pulses, and to receive electrical pulses generated in the individual transducer ~06Z8~Z
elements by reflected ultrasonic waves. The processing of the received signals by the transmit/receive unit 31 occurs during the quiescent period between pulse transmiss-ions. The praticular delay value for each of the variable delay elements 21, 22, ..., 30 is controlled by a con~roller 35, and is determined by the desired scanning ang]e for the array.
Typically, the individual transducer elements of the array 10 are spaced apart by one-half wavelength. This requirement is dictated by the desire for good resolution in the optical sense for the source of reflected waves being imaged. The variable delay lines could provide either continuously variable delay values or could be digitally switched between various discrete delay values. The electronic circuitry required for providing continuously variable delay values is more complicated than circuitry for providing digital switching between discrete delay values, and consequently for most practical applications switching circuitry is provided to enable digital switching between various delay values.
For digitally switched delay lines, the criterion for good image formation is that the phase error produced at any given transducer element be less than + ~/8 where~
is the acoustic wavelength of the ultrasonic wave in the medium through which it is travelling. To satisfy this criterion, the number of delay values (or steps) n into which the dynamic range of a given delay element can be divided should be greater than 2 N sin 6maX, where N i~ the total number of transducer e]ements in the array and ~maxis the maximum steering angle or scanning angle measured from the normal to the array. In deriving this relationship, ~ 106280Z
the focal length f o~ the axra~ is assumed to be large compared to the di`mensions of the array, and the centers of adjacent arra~ eIements are assumed to ~e separated by ~/2.
For a typical array comprising 32 transducer elements and a ma~imum steering angle of 45 degrees, this criterion for good image resolution re~uires that there be 46 or more dela~ steps for each of the delay lines.
Another arrangement known to the prior art for o~taining coherent delays between the transducer elements of an ultra-sonic imaging system is shown in FIG. 3, where the transducer elements 11, 12, 13, ..., 20 are num~ered as in FIG. 2.
The left-end transducer element 11 is coupled to a fixed delay line 21, the right-end transducer element 20 is coupled to a ; fixed delay line 30, and the intervening transducer elements of the array are coupled to separate fixed delay lines 22, 23, ..., respectively. The output signals from adjacent fixed delay lines are coupled, respectively, on either side of a-variable delay element. For example, output signals from fixed delay lines 21 and 22 are coupled res~ectively to the two sides of delay element 40. The fixed delay lines 21, 22 and 23, ..., 30 have differing values, as represented by the differing lengths thereof shown in FIG. 3, when it is intended to scan at an angle to the right of the normal to the array, the delay of the varia~le delay lines 40, 41...48 is greater than the difference of delay of adjacent fixed delay lines so that signals to and from transducer 20 are delayed more than the signals from other transducers to its left. The variable delay elements 40, 41, ..., 48 are controlled by the controller 35. The electronic signal, which is generated by the riaht-end transducer element 20 when an ultrasonic wave front travelling from the right 1~6Z80Z
i~.pin~cs thereon at an ansle ~ w~th respect to the nor~al, passes throug~ the fi~ed delay-line 3~ to the variahle delax element 48. As the wave front cont;nues to travel after impinging the rîght-end transducer element 2~, it impinges in succession upon each transducer element to the left of the right-end element 20. The signal gene:-ated by transducer element lg passes through the fixed dela~y 29 associated there~ith to the circuit line 39 where it is com~ined with the output of the variable delay element 48.
The total delay of the signal from transducer element ~0 produced by the fixed delay line 30 and variable delay element 48 is sufficiently great to allow it to combine in phase with the signal from transducer element lg after it has passed through fixed delay line 29. The combined signals from transducers 19 and 20 are further delayed by additional variable delay elements, and combined with signals from intervening transducers. Finally, the signal contributed by the ]eft-end transducer element 11 is coupled to the circuit line 49 at a point to the left of the 2a variable delay element 40, and combined with the signals contributed by the preceding transducer elements.
For distantly focused ultrasonic heams, i.e., where the focal length of the array is large in comparison with the dimensions of the array, the difference in transmission time or reception time for two adjacent transducer elements is given by the expression ~ = ~d/c) sin ~, where d is the spacing between adjacent transducer element, c is the velocity of the ultrasonic ~ave in the medium through which it travels, and ~ is the steering angle. The maximum difference in delay time between adjacent trans-ducer elements is~max= Cd/c~ 5in ~max-value of delay for the variable delay elements is sufficiently small to fie negl;gible, the difference în delays for adjacent fixed delay elements can be se~t to ~max The maximum required delay of the variable delay elements i5 then 2~raX. The prior art required continuously variable delay elements which ~ere set to exact delay values to match the incident dave front. The present inventor recognizes that it is possible to achieve a minimum num~er of delay steps for each variable delay element in order to satisfy the phase criterion stated above.
Thus for the case where the minimum value of delay ~or the variable delay elements 40, 41,..., 48 is small enough to ~e negligible, the number of delay steps n for each variable delay element required in order to achieve good image resolution according to the criterion stated above i5 n = 4 sin ~ax In deriving this expression, it i5 assumed that the spacing between adjacent trans-ducer elements is ~/2.`
The number of delay val~es required for each variable delay elements 40, 41,... 48, of FIG. 3 is reduced by a factor of N/2 compared to the number of delay values for each variable delay element 21, 22,...30 of FIG. 2.
However, the system of FIG. 3 required the addition of fix-ed delay elements 21, 22, 23,...30. The delay required for the longest of these is at least ~lrmaX where N is the number of transducer elements in the array. The cost and quality of delay lines is determined by the delay-~and-width product. The large number of fixed delay lines and variable delay elements required ~y the prior art systems illustrated by FIGS. 2 and 3 and the requirement for large delay-bandwidth products for these fixed delay lines and some of the variable delay elements contribute 1(~6Z80Z
substantially to the system cost and complexit~. The present inYentiOn proYides a .~u~stantial improvement over these prlor art systems by permï~ting a substantial reduction in the num~er of de'ay values required for ~ach var;able deIay element by elirlinating the need for delay values with the larger delay-bandwldth products, and not requiring any fixed delay lines.
SUMMARY OF THE INVE2~TION
This invention provides'for an ultrasonic imaging system having an array of electromechanical transducers, which can preferentially receive or transmit ultrasonic signals in the desired scanning directions. A system according to this in~-ention has a variable delay element electrically connected to each transducer element of the array, a controller for selecting an appropriate amount of delay of each variable delay element, and switching means for selectively providing various possible electrical connection paths bet~een adjacent delay elements so as to enable the transducer array to preferentially receive or transmit ultrasonic signals propagating either normally to the array or inclined either to the left or to the right with respect to the normal.
It is an object of this invention to enabIe an ultrasonic imaging system having a variable delay element coupled to each transducer e'lement of a scanning transducer array to utilize variable delay elements having a smaller number of delay values per delay element than is required by the prior art.
It is also an object of t~is invention to enable an ultraqonic imaging system to scan at desired angles on either side of the normal to a scanning transducer array, without re~uiring t~at an~ transducer element of the array be coupled to a fixed dela~ element.
rt is th refore an o~ject of this invention to reduce the cost and electronic compl~x;ty of ultrasonic imaging s~stems by reducing t~e numbe:^ of delay elements and the number of delay values per delay element from that required by ultrasonic imaging system cnown to the prior art.
It is also an object of this invention to use delay elements with smaller delay-bandwidth products than known to the prior art.
! It is likewise an object of this invention to use a number of discrete delay values or steps in each variable delay line, rather than a continuous variation in delay, in an ultrasonic imaging system.
Embodiments of the invention will now be described, by way of example.
An embodiment of the present invention is illustrated in block diagram form in FIG. 4. The transducer elements 11, 12, 13, ..., 20 are shown in a linear array, although it is recognized that there are advantages in two-dimensional arrays especially in producing c-scan images. Each transducer element is coupled to a variable delay element. Thus, the left-end transducer element 11 is coupled to a variable delay element 51, the right-end transducer element 20 is coupled to a delay element 60, and the intermediately disposed transducer elements 12, 13 ... of the array are similarly coupled to variable delay elements 52, 53, ..., respectively. An identical switching arrangement is provided between each adjacent pair of Yariable delay elements to control the delay paths of the signals generated in each of the transducer elements when operating in the transmit or receive mode. As illustrated, the switching arrangement between the variafile delay elements 51 and 52 consists of three single-pole, single-throw switches 110, 111 and 112. The positions of the switches can ~e controlled either manually or by means of the controller 35. It is commonly understood that these switches may be transist:ors.
The system shown in FIG. 4 can ~e operating in ei1:her the transmit or receive mode. For the purposes of illustration, the operating of the system will be described in terms of receiving an ultrasonic wave front. The delay arrangements for transmission are identical to those for reception except the direction of travel of the electrical and acoustical signals are reversed. The system will also be described in terms of receiving a parallel wave front, i.e. a focus at infinity. This is done only to simplify the explanation and is not a limitation upon *he system.
For a wave front incident upon the transducer array from the right at an angle with respect to the normal, the r~ght-end transducer element 20 is excited first and the left-end transducer element 11 is excited last as the incident acoustic wave front progresses. Thus, each of the transducer elements in turn from right to left generates an electronic signal in response to the incident wave front. With switch 202 closed and switches 200 and 201 open the signal from the right-end transducer element 20 will pass through the delay element 60 to components of the circuitry indicated to the left thereof in the bloc~ diagram of FIG. 4 for summation with signals generated in time-wise succession ~y the transducer elements to the left thereof.
To consider just two delay elements of the system in response to an ultrasonic wave impinging upon the tranducer array from the right, out~ut of t~e,:'next~to-last delay-element 52 can ~e'conn'ected t~rough closed swltch 112 to the lnput o~ the last dela~ element 51. The delay -element 51 in addition receives an input signal from t~e last transducer element 11. The'output of the delay element 51 is then coupled to the transmit/receive uni1.
31 through,closed swi~tch 50. The signal generated by the transducer element 12 must neces~arily ~e delayed ~y the delay element 52 for a suitable time so as to add in''phase with the signal generated ~y the next succeeding transducer element 11. The signals generated ~y the transducer elements 12 and 11 are then added coherently and are pro-pagated through the delay element 51, and thence through the closed switch 90, to the transmit/receive unit 31 operating in the receive mode.
. Signals generated by any two adjacent transducer elements can be added together in phase and then pro-pagated as an ;nput to a delay element which is coupled to the next succeeding transducer element, ~ust as in the manner described a~ove in connection with the particular adjacent transducer elements 12 and 11. Thus, in FIG. 4, the signals generated ~y the two adjacent transducer elements 13 and 12 can be added together in phase by connecting the output of delay element 53 through closed switch 122 to the input of delay element 52, such that the output of delay element 52 represents the s~mmation of the output from delay element 53 together wit~l the input signal from trans-ducer element 12. The r~quirement that the output from transducer element 13 can ~e added to the output signal from transducer element 12 in phase can be met ~y the selection of an appropriate delay ~alue for the delay ' 106280Z
element 53. ~5 de~cribed aboYe~ thP dela~ Yalue of delay element 52 ;.5 selected to proYlde the proper phasing ~ith the sîgnals from transducer element 11. In like manner, the output of any g;ven element 52, 53,..., or ~O,.as shown in FIG. 4, can be connected to the input to the delay element in the delay path of the signal generated by the next succeeding transducer element in the general direction of propagation of the acoustic wave front.
If it is desired to scan or sweep to the left, a ref.lected wave front arriving from the left would strik_ the left-end transducer element 11 before reaching the adjacent transducer element 12. ~n order to detect such acoustic signals from the left, switch 110 is closed and switches 111 and 112 are open. In this arrangement, the signal generated by transducer element 11 is delayed by the delay element 51, and is then added through closed switch 110 to the signal generated by transducer element 12. The combi:ed output from transducer elements 11 and 12 is then delayed for a suitable time ~y delay element 52, and is subsequently added through closed switch 120 ~switches 121 and 122 being open) to the signal generated by the transducer element 13. As the acoustic wave front impinges in succession upon the other transducer elements located further to the right, the signal generated by each successive transducer element is similarly added to the sum of the signals having passed through the delay elements associated with the preceding transducer elements.
Thus, for a wave front ;mpinging upon the transducer array from the left, ~here the last transducer to be excited is the right-end transducer element 20, the signal generated by the transducer element 20 is added to the integrated ~ 1062802, output signal from the trans~ducers 11, 12, 13, ..., and the total ;ntegrated ~ignal is passed as input to delay element 6Q. The output of deIay element 60 is passed to . the transmi.t/receive unlt 31 operating in the receive mode, upon t~e closure of switch gl ~swi:tch gO ~eing open):.
In the a~ove s~itching arrangement all switches 110, 120,...,200 are closed for steering to the left, and all switches 112, 122,...,202 are closed for steering to the right of the normal. Steering to particular angles is accomplish~d by switching the internal delay lines in the delay elements to different delay values. It is noted, however, that a minimum possible steering angle is deter-mined for the above arrangement because of the irreducible .. minimum delay inherent in each delay element. Therefore, for steering angles smaller than the minimum angle possible with the a~ove arrangement, including steering normal to the transducer array, an alternative switching arrangement is required.
In order to provide a selective response to an acoustic wave front incident normally upon the transducer array, the central switches 111, 121,..., 201 are closed, and the switches 110, 120,...,200 and 112,122,...,202 are opened.
The signal from each transducer element 11, 12, 13.., 20 thereby passes through the delay element Sl, 52, 53,...60 connected directly therewith, so that the output of all the delay elements can be summed and passed directly to the transmit/receive unit 31. By choosing equal delay values for all the delay elements Sl, 52, 53r..o60, the electrical signals generated by an acoustic wave front normally upon the transducer array can all ~e added together in phase and passed to the transmit/receive unit 31.

.

~J~ 106Z802 For steering angles close to or in the direction of the normal, t~e switchin~ arrangement referred to a~ove in w.hich all of the s~itches 111, 121,...,201 are closed may ~e modified so as to provi.de.phase delay appropriate to t~.e selected small angle. The modification consists of opening one or more of the s~itches 111, 121,... ,201 and closi.ng a corresponding set of swîtches 110, 120, ...,200 for steering to the left of the normal, or closing one or more of the switches 112, 122,...,202 for steerins to the right., so that delay elements are connected so as to introduce delay of all signals coming from the left, or from the ri~ht, as the case may be, thereby approximating the phase d~lay required for the selected small steering - angle.
In normal operation, only one of the three switches in each switching arrangement between adjacent delay elements is closed. Thus for the switching arrangement between delay elements, 51 and 52, only one of the switches .110, 111 and 112 is normally closed. One of the switches 90 or 91 will ~e closed depending upon whether it is desired to steer to the right or to the left. Both switches 90 and 91 may ~e closed when steering straight ahead or when focusing is used. In the preferred emhodiment, the transducer elements 11, 12 13~.., 20 can operate in either a transmit mode or a receive mode, with receive mode operation occurring during the quiescent period between transmission pulses. It is to be understood that electronic transmission signals pass through the delay elements in the direction oppcsite to that of the reception 30 signals for any given scanning angle ~.

~ 1062~0Z
The maximum deIay Yalue required for any delay element 51, 52, S3,...,60 is given by the expression ~rmax= ~d/c) sin ~max Assuming that d = ~ 2, and assuming that the phase error of any element of t~e array is less than ~ ~ /8, the number of dela~ steps n~i.e., the maximum delay- valuel is given by the expression n= 2 sin ~max It is thereEore apparent that an imaging system according to the present invention requires a smaller number of delay values, ~y a factor of 2, for each variable delay element than the prior art imaging system shown in FIG. 3.
It is also apparent that the imaging system of the described embodiments does not require fixed delay lines, as were required by the prior art imaging system shown in FIG. 3. Furthermore, the em~odiment does not require the very long total delay values that were required by the - system shown in FIG. 2. This reduction in the number of delay elements and in the number of delay values per delay element provided by the embodiments results in a substantial reduction in the cost Qnd electronic complexity of an ultra-sonic imaging system.
In making the comparisons above, of the various systems, it has been consistently assumed that a phase error of ~ ~ would be accepta~le. Table I indicates the number of de~ay steps re~uired for each variable delay element for the three systems indicated by FIGS.

2, 3 and 4.

-lS-~ 10628~)Z
TABLE
System FIG. 2 46 227 FIG. 3 3 8 FIG. 4 2 4 The second column gives thë num~er of elements required by t~e formulas given above w~ere a phase accuracy of + 1 is required. In some systems it may be desirable to have higher phase accuracy and the third column of Table I indicates the number of delay steps n for each variable delay element when an accuracy of ~ is required. In making these calculations it has been assumed that the array has a total of N = 32 elements and that each element is spaced from its adjacent element by approximately ~/2.
A typical operating frequency might be 2.5 MHz. In this case the minimum step values for + ~ phase error would be 100 nanoseconds and for + ~ 2~ phase error 40 - nanoseconds.
The maxi~um delay ~rmax required of an adjustable delay element 51, 52,... 60 is given by ~d/c)sin ~max With a spacing between transducer elements of d = ~ 2, at an operating frequency of 2.5 MHz and at a maximum scanning angle of ~ max ~r max FIG. S is a schematic diagram illustrating one embodiment of the present inveniton. In this system, amplifiers are used in combination with the delay element to compensate for signal amplitude losses that occur within the delay elements and to provide optimum impedance matching: there~y eliminating undesired reflections or rever~erations wlthin the delay elements. In this system, ::
. ~06Z80Z
, transducer eIe,m,ents~ 11, 12~ Xe coupled through i trans~it-receLve cLrcuîts g2 to transmltters 9~ and receiver preamplifiers 94. T~e'timing signals for the transmitters 93 are deri~ed from tfie'transm~tter controller 36, which in turn is controlled by a master programmer 37. The receiver preamplifiers 54 may also be used to provide gain compensation so that echoes from distant o~ects are amplified to a greater extent than echoes from near~ objects.
This is readily achieved ~y a circuit that increases the , gain of these amplifiers after the transmit pulse has taken place by providing a predetermined gain character-istic as a unction of time. The timing signals for these gain changes may be provided by the master pro;rammer 37.
The preamplifiers 94 may also contain circuits to logarith-mically compress the incoming signals to further compensate for the dif~erences of signal strengths from near~y objects as compared to more distant objects. The outputs of the preamplifiers 9~ are coupled through resistors 95 to ,, the summing junctions 96 of inverting amplifiers 97.
The outputs of the inverting amplifier 97 are coupled through matching resistors 98 to the respective delay lines 51 52..., 60. The gains of these amplifier systems are nominally set to unity through selection of feedback resistors 99. The output of impedances of the delay line 51, 52,..., 60 are matched by means of resistors 101 to the desired characteristic impedances. Any gain losses' in delay lines 51, 52,...,60 can be compensated ~y'feedback resistors 102 of the inverting output amplifiers 103.
The output resistors 104 serve as input summing resistors when, for example, swîtch 110 i5 closed to couple the delayed output of transducer 11 to the signal from transducer 10628(~2 12. The s~itc~ing operation in the xece~ve mode is identical to that described ahove in FIG. 4. The output signal from the receiver 105 i.5 coupled to display 106 ~here it modulates the brightness of its cat~.ode-ray display tube 107. Master programmer 37 controls the X-Y
position of the cathode-ray beam to provide a scan line that has its orientation related to the,direction of the received ultrasonic beam.as determined by the delay line switching combination selected by receiver controller 38.
The preamplifiers 94 pro~ide amplification of the signals from transducer elements 11, 12,..., 20 to achieve a signal level above the noise level of the delay lines and switches, thereby improving the sensitivity of the system., Preamplifiers 94 also provide unidirectional amplification, thereby preventing signals in the delay line circuits from reradiating signals ~hrough the t-ans-ducer elements 11, 12,...,20.
Steering angle information contained in master pro-grammer 37 is fed to the transmitter controller 36, which ' generates timing signals for the transmitters 93 coupled to each transducer 11, 12,...,20. These timing signals cause each transmitter 93 to produce an electrical pulse . in proper timed relationship to cause the corresponding Ultrasonic energy pulses emitted by the transducer elemen.ts ' 11, 12, ..., 20 to add in phase in the direction and focal depth selected by the master programmer 37. Any acoustical impedance discontinuities will cause a part of the ultrasonic energy to be reflected back toward the transducer elements.
The reflected ultrasonic energy is converted to electrical signals by means of the transducer elements 11, 12,..., 20 which are coupled t~rough the transmit-receive coupling net~orks ~2 to th ~xe~mplifiers ~4. The outputs of t~le preampllfïers 54 are coupled to th deIay line networks and s~itc~s as descr;,bed above, ~herein signals arriving from th~ selected direction and focal depth are addec coherentl~ and fed into receiver 105. In the receiver 105, the signals are further amplified and rectified by a radio-freque,ncy detector circuit. The detected signals may be further amplified in a video amplifier contained in receiver lOS and further processed for eXamEle, by a logrithmic amplifier to produce an ou~put signal that is coupled to display 1~6. The output voltage is thus a measure of the reflected signal amplitude from the selected direction and depth; and the time of the occurrance of this signal is directly related to the depth from which the reflection takes place. Thus, by applying this output signal to the display 106 so that it modulates the intensity of the cathode-ray tube 107, a bright spot is formed such that the brightness is related to the - scattering cross section of the object producing the reflected signal. The master programmer 37 applies proper voltages to the X and Y axes of the,cathode-ray tube 107 so that a straight line is drawn that has an orientation related to the direction of the received signal. The time delay-between the output signal from the receiver 105 and the transmitter pulse determines the range of the o~ject.
By sweeping the radial lines of the display at the proper predetermined rate, the distance of the bright spot from the apex gives a direct measure of the range of the scatter-ing object.
In a typical system, the dïspla~ miyht consist of 6~
radial lines, that is, the master proyrammer 37 programs the --lg--:
, ~ 1062802 transmitter controller and receiver controller to sequentially seIect 64 different steering an~les. For one particular angle, the transmitters will all fire within a period of about 6 microseconds. Following that, the receivers will be sensitive to incoming signals for about 200 microseconds. Since the velocity of sound in the human body is approximately 1.5 millimeters/microsecond and since the signal must travel to the point of reflection and back to the transducers, a total range a~Vailable for display is approximately 26 centimeters.
Following this period, there is another period of perhaps 300 microseconds that can be used to allow any residual signals or rever~erations to die down, and si~ultaneously to allow the master progr&~lmer to feed new information to the transmitter and receiver controllers and to allow the receiver controller to select the delay values and switch positions for the next radial scan line. With a total of 64 scan lines, a complete picture may be obtained at a rate of approximately 30 frames/second thereby permitting a r~al time display of moving objects such as the heart.
The above description of the preferred em~odiment of the present invention has been described with specificities which should not be construed as limitations upon the scope of the invention. The scope of the invention is to be construed according to the following claims and their legal equivalents.

Claims (7)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. An ultrasonic imaging system comprising first and second electromechanical transducer elements, first and second electronic variable delay elements, each delay element having an input terminal and an output terminal, said first transducer element being connected to said input terminal of said first delay element and said second trans-ducer element being connected to said input terminal of said second delay element, and switching means for selectively providing electrical connection between either said output terminal of said second delay element and said input terminal of said first delay element, or said output terminal of said first delay element and said input terminal of said second delay element, or said output terminal of said second delay element and said output terminal of said first delay element.

2. The ultrasonic imaging system of claim 1, wherein said first and second transducer elements are disposed adjacent each other in an array of transducer elements.

3. The ultrasonic imaging system of claim 2 wherein said array of transducer elements is linear.

4. The ultrasonic imaging system of claim 2 wherein the transducer elements of said array are capable of transmitting ultrasonic wave pulses in a predetermined direction.

5. The ultrasonic imaging system of claim 4 wherein during the quiescent period between said transmitted pulses, the transducer elements of said array are capable of generating electrical signals in response to a reflected ultrasonic wave incident thereon from said predetermined direction.

6. The ultrasonic imaging system of claim 5 wherein said variable delay elements cause said electrical signals generated in response to said reflected wave from said predetermined direction to be integrated in a coherent summation.

7. The ultrasonic imaging system of claim 6 wherein said coherent summation signals are displayed on a CRT
cathode-ray tube.